A progressive displacement motor (PDM), sometimes referred to as a mud motor or downhole motor; converts hydraulic energy of a fluid such as drilling mud into mechanical energy in the form of rotational speed and torque output, which may be harnessed for a variety of applications such as downhole drilling. A PDM generally comprises a hydraulic drive section, a bearing assembly, and driveshaft. The hydraulic drive section, also known as a power section or rotor-stator assembly, includes a helical rotor disposed within a stator. The driveshaft is coupled to the rotor and is supported by the bearing assembly. Drilling fluid or mud is pumped under pressure between the rotor and stator, causing the rotor, as well as the drill bit coupled to the rotor, to rotate relative to the stator. In general, the rotor has a rotational speed proportional to the volumetric flow rate of pressurized fluid passing through the hydraulic drive section.
As shown in FIGS. 1 and 2, a conventional hydraulic drive section 10 comprises a helical-shaped rotor 30, typically made of steel that may be chrome-plated or coated for wear and corrosion resistance, disposed within a stator 20, typically a heat-treated steel tube 25 lined with a helical-shaped elastomeric insert 21. The helical-shaped rotor 30 defines a set of rotor lobes 37 that intermesh with a set of stator lobes 27 defined by the helical-shaped insert 21. As best shown in FIG. 2, the rotor 30 typically has one fewer lobe 37 than the stator 20. When the rotor 30 and the stator 20 are assembled, a series of cavities 40 are formed between the outer surface 33 of the rotor 30 and the inner surface 23 of the stator 20. Each cavity 40 is sealed from adjacent cavities 40 by seals formed along the contact lines between the rotor 30 and the stator 20. The central axis 38 of the rotor 30 is offset from the central axis 28 of the stator 20 by a fixed value known as the “eccentricity” of the rotor-stator assembly.
During operation of the hydraulic drive section 10, fluid is pumped under pressure into one end of the hydraulic drive section 10 where it fills a first set of open cavities 40. A pressure differential across the adjacent cavities 40 forces the rotor 30 to rotate relative to the stator 20. As the rotor 30 rotates inside the stator 20, adjacent cavities 40 are opened and filled with fluid. As this rotation and filling process repeats in a continuous manner, the fluid flows progressively down the length of hydraulic drive section 10 and continues to drive the rotation of the rotor 30. A driveshaft (not shown) coupled to the rotor 30 is also rotated and may be used to rotate a variety of downhole tools such as drill bits.
As shown in FIG. 3, a simplified version of a conventional downhole drilling system 50 comprises a rig 51, a drill string 52, and a PDM 53 coupled to a conventional drill bit 54. PDM 53 includes hydraulic drive section 10 previously described, a bent housing 56, a bearing pack 57, and a driveshaft 58 coupled to the drill bit 54. The PDM 53 forms part of the bottomhole assembly (BHA) and is disposed between the lower end of the drill string 52 and the drill bit 54. The hydraulic drive section 10 converts drilling fluid pressure pumped down the drill string 52 into rotational energy at the drill bit 54. With force or weight applied to the drill bit 54 via the drill string 52 and/or the PDM 53, also referred to as weight-on-bit (WOB), the rotating drill bit 54 engages the earthen formation and proceeds to form a borehole 60 along a predetermined path toward a target zone. As the drill bit 54 engages the formation, resistive torques generally opposing the rotation of the drill bit 54 and the rotor 30 are applied to the drill bit 54 by the formation. The drilling fluid or mud pumped down the drill string 52 and through the PDM 53 passes out of the drill bit 54 through nozzles positioned in the bit face. The drilling fluid cools the bit 54 and flushes cuttings away from the face of bit 54. The drilling fluid and cuttings are forced from the bottom 61 of the borehole 60 to the surface through an annulus 65 formed between the drill string 52 and the borehole sidewall 62.
Damage and potential failure of the hydraulic drive section of a PDM (e.g., hydraulic drive section 10), may occur for a variety of reasons. One common failure mode is stalling. Referring now to FIG. 4, a plot or graph 80 illustrates the general relationship between the WOB 81 applied to the drill bit 54, the resistive torques 82 applied to the drill bit 54 by the formation, and the rotational speed 83 of the drill bit 54, expressed in terms of revolutions per minute (RPM), for hydraulic drive section 10 previously described. As shown in FIG. 4, hydraulic drive section 10 has a stall torque 82a, which represents the resistive torque 82 applied to the drill bit 54 by the formation that is sufficient to cause hydraulic drive section 10 to stall for the hydraulic drive section 10 in a given condition. In general, the stall torque (e.g., stall torque 82a) for a particular hydraulic drive section (e.g., hydraulic drive section 10) will depend on a variety of factors such as the drive section size and geometry, the stator-rotor lobe configuration, the condition of the seal material at the stator and rotor interface, etc.
Referring still to FIG. 4, the WOB vs. resistive torque curve 85 for hydraulic drive section 10 graphically illustrates, as WOB 81 increases, the resistive torque 82 acting on the drill bit 54 also increases. Although the resistive torque 82 increases, if pumps at the surface maintain a constant volumetric flow rate of drilling fluid through the hydraulic drive section 10 (i.e., the surface pumps can impose sufficient energy into the drilling fluid to overcome the resistive torque 82), then the rotational speed 83a of the drill bit 54 will remain substantially the sane. However, at a sufficient WOB, referred to herein as stall WOB, the resistive torque 82 acting on the drill bit 54 achieves the stall torque 82a. At stall torque 82a, the hydraulic energy of the drilling mud is insufficient to overcome the resistive torque 82, and consequently, rotor 30 stops rotating relative to the stator 20. In other words, at the stall torque 82a, the surface pumps cannot impose sufficient energy into the drilling fluid to overcome the resistive torque 82, and therefore, the drill bit rotational speed 83a drops abruptly to zero. The sudden and near immediate decrease of the rotational speed 83a of the drill bit to zero is typically characterized as a “hard stall”, as opposed to a more gradual reduction in the rotational speed of a drill bit, which may be characterized as a “soft stall”.
Referring now to FIGS. 1-4, in the case of an abrupt or “hard” stall, the drastic change in the rotational speed and momentum of rotor 30 may result in significant and unpredictable impact forces and torques imposed on stator 20 by rotor 30. Such impact forces and torques may cause the mechanical failure of the elastomeric material forming the liner 21 of stator 20. For instance, if the elastomeric material forming liner 21 is loaded beyond its stress and strain limits, portions of the elastomer may tear or break off. Moreover, the stall forces and torques may cause portions of the elastomeric liner 21 to de-bond or become separated (e.g., delaminated) from tube 25. Moreover, as the relative rotational speed of rotor 20 decreases, fluid flow through hydraulic drive section 10 of PDM 53 decreases. As drilling fluid continues to be pumped down the drill string, but less fluid flows through hydraulic drive section 10, a pressure differential across hydraulic drive section 10 increases. If the pressure differential across hydraulic drive section 10 is sufficient, the relatively higher pressure drilling fluid at the upper end of PDM 53 may break the seals between rotor 30 and stator 20 at a relatively high fluid velocity, potentially washing away the elastomeric material forming liner 21. Damage(s) from motor stall often result in a reduction in the power conversion capability of PDM 53, thereby also reducing the rate of penetration (ROP) of drill bit 54 powered by PDM 53.
In general, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth. The time required to drill the well, in turn, is greatly affected by the number of times the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section in order to repair or replace a damaged hydraulic drive section of a PDM. Once the drill string has been retrieved and the rotor and/or stator is repaired or replaced, the entire string must be constructed section by section and lowered into the borehole. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Because drilling costs are typically thousands of dollars per hour, it is thus always desirable to avoid or reduce the likelihood of damaging the hydraulic drive section of a downhole PDM.
Accordingly, there remains a need for apparatus and methods to increase the durability and reliability of a PDM. Such apparatus and methods would be particularly well received if they offered the potential to reduce the likelihood of a “hard” stall and/or limit damage to the elastomeric liner of the stator of the downhole motor assembly as the relative rotational speed of the rotor and stator decreases wider excessive resistive torque from the bit.